The present invention relates generally to musical instrument amplifiers and more particularly to a solid-state amplifier employing a closed loop driver stage and an open loop, even harmonic-rich output stage for providing a tube-like sound desired by many musicians.
Although solid-state amplifiers have become a de facto standard in much of the audio industry, there remains a certain contingent that favors vacuum tube amplifiers. Among those who favor vacuum tube amplifiers are musicians and audiophiles. Vacuum tube amplifiers tend to produce a warm distorted sound characterized by the presence of even harmonics when the amplifier is overdriven. In contrast, conventional bipolar transistor amplifiers tend to have odd harmonic distortion, which is generally considered more harsh or strident than even harmonic distortion. Bipolar transistors, when overdriven, exhibit hard clipping, transforming a sinusoidal input into a sharply defined square wave output. One characteristic of the square wave output is its high frequency content, including considerable power in the odd harmonics. Vacuum tubes, on the other hand, tend to clip in a more rounded manner, somewhat flattening and distorting a sinusoidal input, but without the sharply defined square wave of the bipolar transistor. The vacuum tube amplifier circuit is desired by many because the even harmonic distortion tends to simulate naturally occurring overtones which can be correlated to the musical intervals.
Vacuum tube amplifiers are not without drawbacks, however. First, a conventional vacuum tube circuit requires relatively massive high voltage power supply and vacuum tube circuits tend to generate a lot of unwanted heat. Vacuum tubes also must be replaced from time to time and are more fragile than solid-state amplifiers. Furthermore, due to the wide acceptance of solid-state amplifiers, vacuum tube amplifiers are becoming increasingly costly and obtaining a source of replacement tubes can be a problem for the traveling musician.
There are semiconductor devices which exhibit some tube-like properties, the metal oxide semiconductor field effect transistor (MOSFET) being one of them. MOSFETs generally have a high input impedance and in that regard are similar to vacuum tubes. Although MOSFETs have been used in amplifier circuits in the past, no prior art amplifier has fully exploited the MOSFET device to achieve a tube-like sound.
The present invention exploits the MOSFET device in a hybrid MOSFET/bipolar transistor circuit which provides a desirable tube-like sound in a fully solid state amplifier. When driven into saturation, the amplifier of the invention exhibits a warm tube-like sound characterized by softly clipped or rounded waveforms as contrasted with the sharply defined square wave clipping of a bipolar transistor circuit.
The invention thus provides a device for providing amplification using an electrical signal as an input. The device comprises a closed loop driver stage, such as a bipolar junction transistor amplifier stage, which generates a substantially undistorted signal. In the presently preferred embodiment, the undistorted signal is generated using a differential of the input signal and the output signal of the bipolar junction transistor amplifier, the output signal being comparatively undistorted. The amplifier further comprises an open loop output stage which receives the undistorted signal from the driver stage and generates a distorted output signal rich in even harmonics. The presently preferred embodiment employs a metal oxide semiconductor field effect transistor (MOSFET) amplifier as the output stage.
For a more complete understanding of the invention, its objects and advantages, reference may be had to the following specification, to the accompanying drawings and to the claims.